The biology of pneumocystis is being investigated to determine the role of host defense mechanisms in providing host protection. A. Role of Host Defense Mechanisms. Pneumocystis obtained from rat lung preparation has been applied to in vitro monolayers of mononuclear and polymorphonuclear phagocytes. Pneumocystis tropho zoites can be ingested by either when human cells are assessed. B. Antibody-Parasite Interaction. Pneumocystis trophozoites have been purified, disrupted, and assessed in a variety of columns. Specific protein bands for pneumocystis have been identified. By Western Blot technique rat and human sera from normal and diseased individuals have been assessed for specific binding to these proteins. Monoclonal antibodies to rat and human pneumocystis have been raised and shown to interact with this specific protein. Use of the monoclonal antibodies has shown differences between human and rat pneumocystis that may have important implications for the development of diangostic tests. C. New Therapeutic Agents. In collaboration with DCT, dihydrofolate reductase (DHFR) has been isolated from pneumocystis and toxoplasma, the first time this enzyme has been recovered from these protozoa. A new DHFR inhibitor has been identified which is much more active than previously available drugs. In vitro and animal studies show it to be at least as effective as older drugs. The significance of these studies is that they provide new insights into diagnosis and therapy of this important opportunistic pathogen.